<Project DefaultTargets="CleanAll" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">

    <ItemGroup>
      <Projects Include="**\*.csproj" />
    </ItemGroup>

  <Target Name="CleanAll">
    <MSBuild Projects="@(Projects)" Targets="Clean" StopOnFirstFailure="false" ContinueOnError="true" />
  </Target>

</Project>